Recently, an adhesive sheet has been developed, which has two functions of a dicing tape for fixing a semiconductor wafer to dice the semiconductor wafer into individual chips and a die bonding film (also referred to as a die attach film) for bonding the diced semiconductor chip to a lead frame, a package substrate, or the like or for stacking and bonding together the semiconductor chips in a stacked package.
As such an adhesive sheet, a precut adhesive sheet has been developed in consideration of workability such as attachability to a wafer or attachability to a ring frame before dicing (see, for example, Patent Literature 1).
An example of the precut adhesive sheet is shown in FIGS. 11, 12A and 12B. FIG. 11 is a diagram showing an adhesive sheet wound into a roll, FIG. 12A is a plan view of the adhesive sheet viewed from its pressure-sensitive adhesive film 53 side, and FIG. 12B is a sectional view taken along a line B-B of FIG. 12A. The adhesive sheet 50 includes a release film 51, an adhesive layer 52, and a pressure-sensitive adhesive film 53. The adhesive layer 52 is processed into a circular shape corresponding to the shape of a wafer, and has a circular label shape. The pressure-sensitive adhesive film 53 is formed by removing an area peripheral to a circular portion corresponding to the shape of a ring frame for dicing, and, as shown in the drawings, has a circular label part 53a and a peripheral part 53b surrounding the outside of the circular label part 53a. The adhesive layer 52 and the circular label part 53a of the pressure-sensitive adhesive film 53 are laminated together so that their centers are aligned, and the circular label part 53a of the pressure-sensitive adhesive film 53 covers the adhesive layer 52 and is in contact with the release film 51 around the adhesive layer 52. The pressure-sensitive adhesive film 53 is generally formed by laminating a pressure-sensitive adhesive layer on a base film.
Before wafer dicing, the release film 51 is peeled off from the adhesive layer 52 and the pressure-sensitive adhesive film 53 laminated together, and as shown in FIG. 13, the backside of a semiconductor wafer W is attached onto the adhesive layer 52, and a ring frame F for dicing is adherently fixed to the outer periphery of the circular label part 53a of the pressure-sensitive adhesive film 53. In this state, the semiconductor wafer W is diced. Then, the pressure-sensitive adhesive film 53 is subjected to curing treatment such as irradiation with ultraviolet rays, if necessary, and semiconductor chips are picked up. At this time, the pressure-sensitive adhesive film 53 is separated from the adhesive layer 52 so that the semiconductor chip is picked up with the adhesive layer 52 being attached to its backside. The adhesive layer 52 attached to the backside of the semiconductor chip then functions as a die bonding film when the semiconductor chip is bonded to a lead frame, a package substrate, or another semiconductor chip.
Meanwhile, the pressure-sensitive adhesive film 53 generally covers the adhesive layer 52 and is in contact with the release film 51 around the adhesive layer 52, but there is a case where air remains in a very narrow gap created between the release film 51 and the pressure-sensitive adhesive film 53 due to the thickness of the adhesive layer 52. This phenomenon is more remarkable when the adhesive layer 52 has a thickness as large as 65 μm or more or the pressure-sensitive adhesive film 53 has a high elastic modulus. Such air between the release film 51 and the pressure-sensitive adhesive film 53 does not cause a problem by itself, because the air may move and escape to the outside of the circular label part 53a and does not have a major impact on the physical properties of the pressure-sensitive adhesive film 53.
However, the adhesive sheet 50 is to be placed in a special environment whose temperature varies widely. For example, the adhesive sheet 50 is placed under cold conditions (e.g., −20° C. to 5° C.) during storage and transportation, heated when attached to the semiconductor wafer W, and placed under ordinary temperature conditions when used to process the semiconductor wafer W. Such a temperature change causes major dimensional changes in the release film 51, the adhesive layer 52, and the pressure-sensitive adhesive film 53, and therefore there is a case where air enters between the adhesive layer 52 and the pressure-sensitive adhesive film 53 so that a void is formed. There is a fear that the void causes defective attachment to the semiconductor wafer W and reduces yield in the subsequent semiconductor wafer W dicing process, chip pick-up process, and chip bonding process. It is to be noted that in this specification, the term air refers to air between a pressure-sensitive adhesive film and a release film or between an adhesive layer and a release film, and the term void refers to air between an adhesive layer and a pressure-sensitive adhesive film.
As described above, the void reduces yield and is formed between the adhesive layer 52 and the pressure-sensitive adhesive film 53 and is therefore difficult to remove, which is a very serious problem for the adhesive sheet 50. Further, in light of the principle of void formation, air is more likely to develop into a void when its amount is larger. For this reason, the amount of air is preferably reduced.
As a measure to suppress void formation, a laminated sheet has been disclosed (see, for example, FIGS. 1 and 2 in Patent Literature 2), which has through holes provided so as to penetrate a base film and a pressure-sensitive adhesive layer of a pressure-sensitive adhesive film in the thickness direction of the pressure-sensitive adhesive film. By providing such through holes in the pressure-sensitive adhesive film, it is possible to prevent void formation between an adhesive layer and the pressure-sensitive adhesive film, and therefore it can be expected that the occurrence of defective attachment to a semiconductor wafer will be effectively suppressed.